1. Field of the Invention
This invention pertains to radio antennas. More particularly, this invention pertains to an array of such antennas which is so ordered as to create an optimally sited fixed antenna network which is suited to use in, e.g. an AMPS system.
2. Description of the Prior Art
Mobile telephones, such as those which are installed in automotive vehicles and the like, must be linked to the telephone switching system by means of fixed antennas. In regions where radio traffic is dense, it is desirable to provide a large number of users with mobile telephone service without co-channel interference taking place between radio transmissions which are transmitted simultaneously.
To this end, various antenna systems have been developed. In an article entitled The Cellular Concept, which article appears in The Bell System Technical Journal of January 1979, a plurality of such systems are discussed and a new system denoted AMPS (an acronym for Advanced Mobile Phone Service) is set forth.
In the system which is newly-disclosed therein, use is made of a cellular antenna array in which a basic hexagonal cell (which may have a varying size) is equipped with three sector antennas. Each such sector antenna has a 120.degree. directional characteristic, and the three antennas are so oriented as to provide 360.degree. coverage from each cell. Moreover, in the system disclosed in this reference, each such cell transmits and receives over frequencies within a given frequency group, and the frequency groups are relatively numerous.
At page 22 of this reference, a co-channel reuse ratio is computed, which co-channel reuse ratio is used as an index of transmission quality. This co-channel reuse ratio increases with an increasing number of frequency groups utilized, and decreases with a decreasing number of frequency groups utilized.
The AMPS system disclosed in this reference thus has two major disadvantages. Firstly, it is necessary to construct three antennas at each cell site. Secondly, it is necessary to utilize a relatively large number of frequency groups in order to achieve the requisite transmission quality, and it is thus necessary to utilize a relatively wide portion of the available frequency spectrum. Thus, it would be desirable to provide an antenna array which would not require a plurality of antennas for each cell site, and which would operate using a lower number of frequency groups.